He made himself useful to Pope Martin V (1417–31) and was quickly elected to succeed him. Eugene was crowned as Eugene IV at St. Peter's, March 11, 1431. By a written agreement made before his election he agreed with the cardinals to distribute to them one-half of all the revenues of the Church and promised to consult with them on all questions of importance, both spiritual and temporal. Upon taking the Papal Chair, Eugene IV took violent measures against the numerous Colonna relations of his predecessor, Pope Martin V (Ottone Colonna), who had rewarded his numerous clan with castles and lands. This at once involved him in a serious contest with the powerful house of Colonna that nominally supported the local rights of Rome against the interests of the Papacy. A truce was soon arranged.
But by far the most important feature of Eugene IV's pontificate was the great struggle between the Pope and the Council of Basel, commonly referred to as the Council of Florence, (1431–39), part of the historic Conciliar movement. On July 23, 1431, his legate, Giuliano Cesarini, opened the council, which had been convoked by Martin V, but, distrustful of its purposes and emboldened by the small attendance, the pope issued a bull on December 18, 1431, dissolving the council and calling a new one to meet in eighteen months at Bologna. The council resisted this premature expression of papal prerogative, as it appeared to the majority of them. Eugene IV's action gave some weight to the contention that the Curia was opposed to any authentic measures of reform. The council refused to dissolve; instead they renewed the resolutions by which the Council of Constance had declared a council superior to the Pope, and cited Eugene IV to appear at Basel. A compromise was arranged by Sigismund, Holy Roman Emperor, who had been crowned emperor at Rome on May 31, 1433. By its terms the Pope recalled his bull of dissolution, and, reserving all the rights of the Holy See, acknowledged the council as ecumenical (December 15, 1433). The pope agreed to name presidents to lead the council on his behalf.
These concessions also were due to the invasion of the Papal States by the former Papal condottiero Niccolò Fortebraccio and the troops of Filippo Maria Visconti led by Niccolò Piccinino, in retaliation to Eugene's support to Florence and Venice against Milan (see also Wars in Lombardy). This situation led also to establishment of an insurrectionary republic at Rome, controlled by the Colonna family. In early June, disguised in the robes of a Benedictine monk, he was rowed down the center of the Tiber, pelted by stones from either bank, to a Florentine vessel waiting to pick him up at Ostia. The city was restored to obedience by Giovanni Vitelleschi, the militant Bishop of Recanati, in the following October. In August of 1435 a peace treaty was signed at Ferrara by the various belligerents. The Pope moved to Bologna in April of 1436. His condottieri Francesco Sforza and Vitelleschi in the meantime reconquered much of the Papal States. Traditional papal enemies such as the Prefetti di Vico were destroyed, while the Colonna were reduced to obedience after the destruction of their stronghold in Palestrina (August, 1436).
Meanwhile the struggle with the council sitting at Basel broke out anew. Eugene IV at length convened a rival council at Ferrara on January 8, 1438, and excommunicated the prelates assembled at Basel. The result was that the Council of Basel suspended him on January 24, 1438, then formally deposed him as a heretic on June 25, 1439, and in the following November elected the ambitious Amadeus VIII, Duke of Savoy, antipope under the name of Felix V. The conduct of France and Germany seemed to warrant this action, for Charles VII of France had introduced the decrees of the Council of Basel, with slight changes, into France through the Pragmatic Sanction of Bourges (July 7, 1438), and the Diet of Mainz had deprived the Pope of most of his rights in the Empire (March 26, 1439).
At Florence, where the council of Ferrara had been transferred on account of an outbreak of the plague, a union with the Eastern Orthodox Church was effected in July, 1439, which, as the result of political necessities, proved but a temporary bolster to the papacy's prestige.
This union was followed by others of even less stability. Eugene IV signed an agreement with the Armenians on November 22, 1439, and with a part of the Jacobites in 1443, and in 1445 he received the Nestorians and the Maronites. He did his best to stem the Turkish advance, pledging one-fifth of the papal income to the crusade which set out in 1443, but which met with overwhelming defeat at Varna. Cardinal Cesarini, the papal legate, perished in the rout.
His rival, Felix V, meanwhile, obtained small recognition, even in the Empire. Eventually Frederick III, king of the Romans, moved toward acceptance of Eugene. The king's ablest adviser, the humanist Aeneas Sylvius Piccolomini, who was later to be Pope Pius II, made peace with Eugene IV in 1442. The Pope's recognition of the claim to Naples of King Alfonso V of Aragon (treaty of Terracina, signed by Eugene at Siena somewhat later) withdrew the last important support from the council of Basel. In 1442 Eugene, Alfonso and Visconti sent Niccolò Piccinino to reconquer the March of Ancona from Francesco Sforza, but the defeat of the allied army at the Montolmo pushed the Pope to reconcile with Sforza.
His protests against the Pragmatic Sanction of Bourges were ineffectual, but by means of the Concordat of the Princes, negotiated by Piccolomini with the electors in February, 1447, the whole of Germany declared against the antipope. This agreement was finalized only after Eugene's death.
Although his pontificate had been so stormy and unhappy that he is said to have regretted on his deathbed that he ever left his monastery, nevertheless Eugene IV's victory over the council of Basel and his efforts on behalf of church unity contributed greatly to the breakdown of the conciliar movement and restore the papacy to the dominant position it had held before the Western Schism (1378–1417).
Eugene IV was dignified in demeanour, but inexperienced and vacillating in action and excitable in temper. Bitter in his hatred of heresy, he nevertheless displayed great kindness to the poor. He laboured to reform the monastic orders, especially the Franciscans, and was never guilty of nepotism. Although austere in his private life, he was a sincere friend of art and learning, and in 1431 he re-established the university at Rome. Eugene was buried at Saint Peter's by the tomb of Pope Eugene III, the former pupil of Bernard of Clairvaux.Later his tomb was transferred in San Salvatore in Lauro-a parish church on the other bank of the Tiber river.